<?php

$ss = '{
  "items": [
    {
      "extension_attributes": {},
      "order_item_id":155,
      "qty": 1
    }
  ],
  "notify": true,
  "appendComment": true,
  "comment": {
    "extension_attributes": {},
    "comment": "string",
    "is_visible_on_front": 0
  },
  "tracks": [
    {
      "extension_attributes": {},
      "track_number": "1Y-9876543210",
      "title": "United Parcel Service",
      "carrier_code": "us"
    }
  ],
  "packages": [
    {
      "extension_attributes": {}
    }
  ],
  "arguments": {
    "extension_attributes": {}
  }
}';

$js = json_decode($ss,true);

print_r($js);